Quantum crypto still not proven, claim Cambridge experts

Two killjoy researchers from the University of Cambridge have cast doubt on whether quantum cryptography can be regarded as ‘provably secure’ – and are asking whether today’s quantum computing experimentation is demonstrating classical rather than quantum effects.

Computer scientists Ross Anderson and Robert Brady have published their discussion at Arxiv, here. In the paper, they examine two key issues in quantum research. As well as looking at the cryptography question, they also examine why quantum computing research is finding it hard to scale beyond three qubits.

“Despite the investment of tremendous funding resources worldwide, we don't have working testbeds; we're still stuck at factoring 15 using a three-qubit algorithm”, the paper notes. It suggests that current experiments have not yet proven that “local realism” (that is, classical behaviour without the “spooky action at a distance” that so bothered Einstein) is violated.

The question the paper seeks to raise (not answer: although the paper proposes experiments to test the theory) is this: can current experiments that appear to demonstrate purely quantum behaviours actually be explained by classical physics?

A metaphorical explanation is given by the example of a droplet that is phase-locked with a wave, as illustrated in this YouTube video:

Such experiments “show clear phenomena corresponding to those of quantum mechanics, including single-slit diffraction, double-slit diffraction, quantised energy levels and tunneling through a barrier.”

Entanglement says that two entangled particles (say, photons) will reflect each others’ state at a rate greater than the speed of light.

Anderson and Brady are asserting that experiments conducted to date observe coherence between distant particles, but fail to eliminate the possibility that the two particles are responding to an identical third stimulus – like the bouncing droplet in the video. In other words: two boats on a lake, bouncing on the waves, aren’t demonstrating quantum physics, they’re responding to the same ripples.

Hence their doubts about cryptography: “As the experiments done to test the Bell inequalities have failed to rule out a classical hidden-variable theory of quantum mechanics such as the soliton model, the security case for quantum cryptography based on EPR [The Register - Einstein – Podolsky Rosen] pairs has not been made.”

In a statement that’s bound to spark hot debate among both theorists and experimentalists, the paper asserts that “experimental work which appeared to demonstrate a violation of Bell's inequalities might not actually do so; regardless of whether it is a correct description of the world, it exposes a flaw in the logic of the Bell tests.”

El Reg is completely unqualified to ask whether the new experiments proposed by the paper will settle the debate, but it might be a good time for physics-watchers to grab the popcorn. ®